Cost of long distance lightwave communication systems, whether terrestrial or undersea, is closely related to the unrepeatered distance spanned by the optical fiber transmission medium. As the distance increases, the relative cost of a system generally decreases with respect to cost of installation and cost of maintenance.
Actual and proposed long distance lightwave communication systems described in the literature strive for long unrepeatered links of optical fiber by employing lumped amplifiers at the end of each fiber span. Each discrete amplifier linearly boosts the optical signal power supplied to the next span of fiber as shown in FIG. 1 in much the same manner as conventional electronic amplifiers for analog coaxial-cable systems. See Optical Fiber Telecommunications II, edited by S. E. Miller et al., pp. 819-22 (Academic Press:1988). Optical isolators are generally employed with each amplifier to avoid feedback effects. Since isolators are unidirectional devices, the resulting lightwave system is also unidirectional.
Optical amplification has been achieved by both semiconductor amplifiers and fiber amplifiers. Semiconductor amplifiers utilize stimulated emission from injected carriers to provide gain whereas fiber amplifiers provide gain by stimulated Raman or Brillouin scattering or fiber dopants such as molecular D.sub.2 or Er.sup.3+.
While these amplifiers are simpler, less expensive alternatives to regenerative optoelectronic repeaters, it cannot be overlooked that their use adds noise which accumulates from one amplification section to the next. In response to this problem, distributed amplification systems employing stimulated Raman scattering and stimulated Brillouin scattering were observed as alleviating the problem. However, it has been determined that such methods for achieving distributed amplification on a substantially uniform basis are limited for reasonable pump powers on bidirectionally pumped fibers to fiber lengths on the order of 50 km. This can be understood from the fact that the pump power decays exponentially according to the loss coefficient of the optical fiber at the pump wavelength.
At the present time, most telecommunication system designers specify a required optical fiber span between amplifiers of 100 km. or more. To achieve transmission over such long spans with cascaded discrete amplifier stages, it is possible to increase the optical signal power launched into the fiber at the transmitter to overcome the intrinsic loss of the longer fiber. However, such an approach causes significant signal intensity variations in the transmission fiber which lead to serious problems with nonlinear effects in the fiber itself and, possibly, to problems with saturation of the signal amplifiers.